Exhaust gas treatment devices are used on automobiles to reduce atmospheric pollution from engine emissions. Examples of widely used exhaust gas treatment devices include catalytic converters and diesel particulate traps.
A catalytic converter for treating exhaust gases of an automotive engine includes a housing, a fragile catalyst support structure for holding the catalyst that is used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen, and a mounting mat disposed between the outer surface of the fragile catalyst support structure and the inner surface of the housing to resiliency hold the fragile catalyst support structure within the housing.
A diesel particulate trap for controlling pollution generated by diesel engines generally includes a housing, a fragile particulate filter or trap for collecting particulate from the diesel engine emissions, and a mounting mat that is disposed between the outer surface of the filter or trap and the inner surface of the housing to resiliently hold the fragile filler or trap structure within the housing.
The fragile structure generally comprises a monolithic structure manufactured from a frangible material of metal or a brittle, ceramic material such as aluminum oxide, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These materials provide a skeleton type of structure with a plurality of gas flow channels. These monolithic structures can be so fragile that even small shock loads or stresses are often sufficient to crack or crush them. In order to protect the fragile structure from thermal and mechanical shock and other stresses noted above, as well as to provide thermal insulation and a gas seal, a mounting mat is positioned within the gap between the fragile structure and the housing.
The mounting mat materials employed should be capable of satisfying any of a number of design or physical requirements set forth by the fragile structure manufacturers or the exhaust gas treatment device manufacturers. For example, the mounting mat material should be capable of exerting an effective residual holding pressure on the fragile structure, even when the exhaust gas treatment device has undergone wide temperature fluctuations, which causes significant expansion and contraction of the metal housing in relation to the fragile structure, which in turn causes significant compression and release cycles for the mounting mats over a period of time.
Ceramic and metallic substrates used in exhaust gas treatment devices are most often mounted within a metal housing with an inorganic fiber based mounting mat. This mounting mat material may contain only inorganic fibers. However, the mounting mat material may also contain other types of fibers, organic binders, inorganic binder and intumescent materials.
The mounting mat must function across a wide range of operating temperatures to effectively hold the substrate in position. Substrates are subjected to axial forces acting on the substrate due to vibrations. The mounting mat also compensates for the fact that the metal housing expands more or less than the substrate itself. Various exhaust gas treatment devices operate throughout a temperature range of ambient conditions 20° C. to about 1200° C. Therefore, mounting mats must provide robust holding pressure performance across this wide temperature range.
As low temperature applications become more prevalent either from more efficient engine design or an increase in popularity of diesel powered vehicles, the desire for mounting mats that perform well at both low and high temperatures has increased.
For low temperature exhaust gas treatment device applications, such as diesel particulate traps or diesel catalyst structures, while these devices do not reach the temperatures provided in high temperature catalytic converters, the weight of the fragile structure and loading techniques employed require the mounting mat have a effective residual minimum holding pressure. In these applications, a higher minimum shear strength for the mounting mat of at least about 25 kPa is preferably achieved to prevent that fragile structure from being dislodged and damaged. The coefficient of friction of these mat products in such high G-load applications with heavy substrates is still approximately 0.45 in the in-use condition. Therefore, a mounting mat for this type of application should have an effective residual minimum holding pressure after 1000 cycles of testing at a temperature of about 300° C. of at least about 50 kPa.
In low temperature applications, such as turbocharged direct injection (TDI) diesel powered vehicles, the exhaust temperature is typically about 150° C. and may never exceed 300° C. It has been observed in the field that catalytic converters, that are assembled with typical intumescent mats, fail with an unexpectedly high frequency.
One reason for these failures is that the exhaust temperature is too low to expand the intumescent material, typically vermiculite particles. This has even been found in converters that have been pre-heated to about 500° C. to pre-expand the intumescent particles. When subsequently used in the low temperature applications, the mats fail to provide sufficient pressure against the fragile structure and thus fail. At temperatures above 350° C., the intumescent particles expand and increase the holding force of the mat against the fragile structure.
Flexibility is achieved by impregnating a mounting mat with various organic binders. However, exhaust gas treatment devices have suffered from poor low temperature performance (<300° C.) due to the presence of organic binder in the mat products, which degrade and cause a loss in the holding force. From room temperature to about 200° C. the loss in holding force is gradual. However, the loss in holding force is rapid from about 200° C. to about 250° C.
Previous attempts have been made at improving the low temperature performance of mounting mat materials for exhaust gas treatment devices. One such attempt involves including expanding particles in the mounting mat which expand (that is, increase in volume) throughout the temperature range where the organic binder has a negative impact. Unfortunately, such expanding particles continue to expand at temperatures well above the temperatures at which the organic binders exhibit their negative impact on mat performance.
What is needed in the industry is a flexible mounting mat for exhaust gas treatment devices which can be easily installed and which can function across a wide range of inlet gas temperatures without a significant loss in mat thickness and corresponding holding pressure performance.